A few people may already have heard about this famous incident to which no answers were found which could explain all of what happened.
It is a very strange case, however the reason for the post is because the Russian authorities are to reopen the investigation.
Whether that will bring any new answers or new theories I am uncertain, but very interesting none the less.

Why is a grizzly bear attack is not a reasonable explanation? Sometimes they wake up from winter slumber early and are very dangerous in such situations.

The Russian police generally and most certainly in that location would be very well versed in bear attacks and would have seen the signs of bears if there were any. But apparently not and that is another of the many factors in this mystery.
I hope they've kept the evidence well preserved.